WASHINGTON — U.S. airline delays are at their highest level in at least 13 years, and analysts say fliers can expect more of the same for the rest of the summer.

The Department of Transportation on Monday said the industry’s on-time performance in the first six months of the year was its worst since the agency began gathering comparable data in 1995. In June, nearly a third of domestic flights on major U.S. airlines were late. Part of the explanation for the worsening delays is that demand for air travel is rising, both on major airlines and on smaller regional carriers. In addition, the government said weather-related delays in June were up 7 percent from a year ago. Reports of mishandled baggage and complaints filed with the government also rose.

Airline consultant Robert Mann said U.S. carriers improved their financial health in recent years by relying more on small 40- to 80-seat jets that are easier to fill up, and can be more profitable because there are fewer empty seats. However, this strategy also leads to more crowded skies and runways in a system “that was already saturated,” Mann said.

Regional carriers served 155.7 million passengers last year, up 38 percent from 2003 levels, according to the Washington-based Regional Airline Association. Those carriers, which include Mesa Air Group Inc. and Delta Air Lines Inc.’s Comair subsidiary, operated at nearly 74 percent of capacity on average last year, up from 66 percent four years ago.

For June, U.S. airlines’ on-time arrival rate was just above 68 percent, compared with roughly 73 percent a year earlier, according to Department of Transportation data. So far in 2007, nearly 25 percent of flights on the 20 largest carriers have arrived late, the agency said. Travelers on SkyWest Inc.’s Atlantic Southeast Airlines, a regional carrier for Delta, had it worst in June, as about 56 percent of flights arrived on time, and five of that airline’s flights were late 100 percent of the time. AMR Corp.’s American Airlines was barely better, with an on-time arrival rate of about 58 percent. US Airways Group Inc. had an on-time rate of about 62 percent.

The airline industry blames the increased delays on a lack of a modern satellite-based air traffic control system, combined with increasing demand.

“We’re not surprised by the numbers,” said David Castelveter, a spokesman for the Air Transport Association, the airlines’ trade group. “We have been saying for some time: It’s going to get worse before it gets better.”